This invention relates to a process for hydrogenating olefinically unsaturated polymers.
Various types of organic polymers contain olefinic unsaturation, that is, unsaturation of the type xe2x80x94Rxe2x80x2Cxe2x95x90CRxe2x80x2xe2x80x94 or xe2x80x94Rxe2x80x2Cxe2x95x90CRxe2x80x22, wherein Rxe2x80x2 is hydrogen or an organic radical. The most notable of these are polymers of conjugated dienes, such as poly(butadiene), which contain olefinic unsaturation sites either in the polymer chain itself or pendant thereto. For various reasons, it is sometimes desired to eliminate this unsaturation. The olefinic unsaturation is subject to attack by oxidants, heat, radiation or a combination thereof. This causes the polymer to perform poorly in applications where it is exposed to such conditions.
Thus, various methods of removing this unsaturation through hydrogenation have been devised. These methods generally involve reacting the polymer with a hydrogenating agent, such as hydrogen, in the presence of a metal catalyst.
One such method involves the use of a carrier-supported heterogeneous catalyst in which a metal, such as nickel, platinum or palladium, is supported on a carrier, such as carbon, silica or alumina. Heterogeneous catalysts are often lower in activity than homogeneous catalysts and also often require higher temperature and pressure conditions in order to achieve acceptable reaction rates. The higher temperatures increase energy consumption, thus cost, and can cause the polymer to decompose or the reaction system to gel. Furthermore, a larger amount of heterogeneous catalyst is usually needed on a weight basis, particularly when a high molecular weight polymer is hydrogenated. Also, heterogeneous catalysts are often not selective and can sometimes catalyze hydrogenation of other portions of the polymer, particularly aromatic rings, which may be undesirable for some applications.
Because of the problems with heterogeneous catalysts, homogeneous catalysts have been developed. Homogeneous cobalt and nickel catalysts have been used. U.S. Pat. Nos. 4,668,773; 4,716,257 and 4,801,666 disclose the use of certain lanthanide catalysts in hydrogenation reactions. In addition, certain titanium complexes have also been tried. U.S. Pat. No. 3,920,745, for example, describes the use of certain dicyclopentadienyl carbonyl acetylene titanium compounds in hydrogenating olefins and diolefins. U.S. Pat. No. 5,017,660 describes the hydrogenation of butadiene and styrene-butadiene polymers using di-p-tolylbis-(cyclopentadienyl)titanium (IV) as the catalyst. In U.S. Pat. No. 4,501,857 and U.S. Pat. No. 4,980,421 certain bis(cyclopentadienyl) titanium compounds were used with lithium components in hydrogenation. In U.S. Pat. No. 4,673,714 certain bis(cyclopentadienyl) titanium compounds were found not to require lithium compounds as cocatalysts. Similarly, U.S. Pat. No. 5,206,307 describes the use of various bis(cyclopentadienyl)titanium (IV) compounds to hydrogenate unsaturated polymers in the presence of an alkyl benzoate promoter. U.S. Pat. No. 5,789,638 discloses the use of certain monocyflopentadienyl titanium and zirconium compounds in hydrogenation of copolymers of conjugated dienes.
Despite the development of homogeneous catalysts, several problems remain. Many of the catalysts are colored and must be removed from the product polymer. Others are toxic or present environmental problems. Still others do not selectively hydrogenate olefinic unsaturation, or are not sufficiently active to be used economically.
Thus, it is desirable to have a catalyst which provides reasonably facile hydrogenation under mild or moderate conditions, leaves colorless residues and which avoids known environmental or disposal problems. It is further desirable that the catalyst provides for selective hydrogenation. This is particularly the case where the polymer being hydrogenated contains, in addition to the olefinic unsaturation, other sites that are also subject to hydrogenation or reduction. A prominent example of such a polymer is a copolymer of a diene, such as butadiene, and a vinyl aromatic, such as styrene. Often it is desired to remove the residual unsaturation that is inherent in diene polymers without hydrogenating the rings of the vinyl aromatic monomers. In such instances, the catalyst desirably is highly selective for the olefinic unsaturation, yet provides for facile reaction.
Further, it would be desirable to hydrogenate a polymer, copolymer or mixture thereof having olefinic unsaturation while minimizing cleavage or degradation of the polymer backbone, thus retaining polymer molecular weight through the hydrogenation process.
This invention is a process for selectively hydrogenating at least one polymer or copolymer or mixture thereof containing olefinic unsaturation. The process comprises reacting or contacting the polymer, copolymer or mixture thereof with a hydrogenating agent in the presence of a catalytic amount of a divalent Group IV metal compound, which is represented by the structure:
CpCpxe2x80x2Mxe2x80xa2Dxe2x80x83xe2x80x83Formula 1
wherein M is titanium, zirconium or hafnium, Cp and Cpxe2x80x2 are each substituted or unsubstituted cyclopentadienyl ligands, which are optionally the same or different from each other and optionally Cp and Cpxe2x80x2 include substituents which form a bridging group connecting Cp and Cpxe2x80x2, and D represents a conjugated, neutral diene, which is coordinated via xcfx80 complexation to the M atom.
According to this method, a facile hydrogenation of the olefinic unsaturation of a polymer is achieved under mild to moderate conditions. Moreover, the hydrogenation is selective, with aromatic groups being minimally affected in the process. Thus, the process is particularly suited for hydrogenating a copolymer of at least one conjugated diene and at least one vinyl aromatic monomer. Advantageously cleavage and degradation of the polymer backbone is minimized relative to cleavage or degradation observed using other hydrogenation catalysts as demonstrated by retention of polymer molecular weight through the hydrogenation process.